In "By the Waters of Babylon," what are the old books and writings John learns to read?

We can infer that these are books and writings left from the civilization that was destroyed--our civilization, as we discover at the story's conclusion. John explains that he has learned much knowledge and many secrets from reading, and he has learned how to write. Only priests and those who will become priests know of the books and writings. Others believe that the priests "do all things by chants and spells." One of the many secrets John has learned is "how to stop the running of blood from a wound," a basic first-aid technique. A clear indication that these are books that belonged to the lost civilization is that John knows how "the gods" dressed. He says, "We know how they were dressed from the book . . . ."